I needed to insert new line after existing one in multiple similar xml files.

In Visual Studio open "replace in files" dialog and tick "use regular expression"

In "find" escape "/" as "\/"

     <param name="ExistingKey" value="False" \/>

In "Replace" repeat original string, then add \n and add a new string

     <param name="ExistingKey" value="False" />\n  <param name="NewKey" value="new value" />

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